This invention relates to a drive circuit for light-emitting diodes (hereafter referred to as LEDs) used as digital display elements in a digital instrument, and more particularly to such a drive circuit of a dynamic drive type.
The recent development of digital techniques has put into practice the with digital displays (for example, of time in watches, or of frequency being received in radio receivers or tranceivers) by the use of 7-segment display units.
For driving such 7-segment display units the two methods of static drive and dynamic drive (pulsed energization) are generally used. The static drive type is usually recommended in the case where one or two digits are to be displayed, while the dynamic drive type is preferred in terms of economizing circuit elements in the case where two or more digits are to be displayed. This is because the number of LED drive switches (and hence their associated wiring circuits) required for the display of N digits each having seven LEDs for 7 segments and one LED for a decimal point is 8N in the static drive method and (8+N) in the dynamic drive method.
In a drive circuit employing such a dynamic drive method, the corresponding segments or LEDs of the respective digits are connected in common and there are provided segment switches for selecting the segments and character switches for selecting the digits. Each segment is connected in series with the associated segment switch and character switch, and the segment switches and the character switches are controlled in a time-division manner. Accordingly, there is a drawback that the current flowing through the LED upon the turn-on of the segment or character switch changes pulsewise or stepwise so that the higher harmonics contained in the pulse-like current cause reception disturbance in a receiver.